1. Field of the Invention
The present invention relates to an air-fuel ratio feedback control system in an internal combustion engine having a single air-fuel ratio sensor downstream of a three-way reducing and oxidizing catalyst converter in an exhaust gas passage.
2. Description of the Related Art
Among known air-fuel ratio feedback control systems using air-fuel ratio sensors (O.sub.2 sensors), there exists a single air-fuel ratio sensor system, i.e., having only one air-fuel ratio sensor. Note, in this system the air-fuel ratio sensor is disposed either upstream or downstream of the catalyst converter.
In a single air-fuel ratio sensor system having an air-fuel ratio sensor upstream of the catalyst converter, the air-fuel ratio sensor is disposed in the exhaust gas passage near to a combustion chamber, i.e., near the concentration portion of an exhaust manifold. In this system, however, the output characteristics of the air-fuel ratio sensor are directly affected by a non-uniformity or non-equilibrium state of the exhaust gas. For example, when the air-fuel ratio actually indicates a rich state, but oxygen is still present, the output characteristics of the air-fuel ratio sensor fluctuate. Also, in an internal combustion engine having a plurality of cylinders, the output characteristics of the air-fuel ratio sensor are also directly affected by differences in individual cylinders, and accordingly, it is impossible to detect the mean air-fuel ratio for the entire engine, and thus the accuracy of the control of the air-fuel ratio is low.
On the other hand, in a single air-fuel ratio sensor system having an air-fuel ratio sensor downstream of the catalyst converter, the non-uniformity or non-equilibrium state of the detected exhaust gas has little or no effect, and thus the mean air-fuel ratio for the engine can be detected. In this system, however, due to the capacity of the catalyst converter, the response characteristics of the air-fuel ratio sensor are lowered, and as a result, the efficiency of the catalyst converter cannot be properly exhibited, and thus the HC, CO and NO.sub.x emissions are increased.
To solve the above problems, the following method, for example, is known. Namely, the actual air-fuel ratio is adjusted by a self-oscillating term, and the mean value thereof, i.e., a coarse-adjusting term, is controlled in accordance with the output of the air-fuel ratio sensor disposed downstream of the catalyst converter.
Nevertheless, this method cannot eliminate the increase of HC, CO and NO.sub.x emissions, because a convergence error in the stoichiometric air-fuel ratio occurs due to a phase-difference between the input and the output of the exhaust gas, caused by a low response of the air-fuel ratio sensor.
To solve the above problem, the present inventors have suggested a method of avoiding an overcompensation, which inhibits the gradual change of the coarse-changing term when the time for which the output of the air-fuel ratio sensor is inverted becomes shorter than a predetermined time, because this state can be shown as the actual air-fuel ratio converges on the stoichiometric ratio (see Japanese Unexamined Patent Application (Kokai) No. 2-230934 published on Sept. 13, 1990).
This method, however, cannot avoid a large deviation of the coarse-adjusting term from the stoichiometric ratio when the performance of the catalyst converter, i.e., the O.sub.2 storage effect, is weakened. In this state, HC, CO and NO.sub.x in the exhaust gas cannot be absorbed by the catalyst converter, large fluctuations of the measurement of the exhaust gas by the air-fuel ratio sensor disposed downstream of the catalyst converter occur, in the same way as when the air-fuel ratio sensor is disposed upstream of the catalyst converter. As a result, the time for which the output of the air-fuel ratio sensor is inverted becomes shorter, whereby the gradual change of the coarse-adjusting term is inhibited.